Time is returned along with costs
Changed stopping criterion

Iterations are now stopped when all coefficients in the estimated
precision matrix change less than the tolerance between two
iterations.
This gives kind of a guaranteed number of decimals in the estimated
precision matrices.
Minor bug fix.
Probing of group_sparse_covariance

Added keyword "probe_function" to group_sparse_covariance. This is a
way to provide a function that is called after each iteration. See
aspect-oriented programming.

Updated the standard message displayed for each iteration, to show the
max norm of the difference between two estimation of the precision
matrices.
Remove "return_costs" keyword

This keyword is useless because the same feature can be obtained using
a probe.
Code cleanup

Almost every "dtype" keyword argument has been removed.